The present invention relates to distance measuring apparatus and more particularly to FMCW altimeters and ranging systems.
As is well known, there are generally two types of distance measuring systems, one known as pulsed radar and the other as FMCW. In the pulsed radar technique, a series of RF pulses are transmitted towards the desired target and the receiver is operated to detect the return of the transmitted pulses that are reflected from the target. The time delay between the transmitted and received pulses is proportional to the distance to the target. By accurately controlling the transmission and detection of such pulses, highly accurate information can be obtained with respect to distance to the target.
In an FMCW distance measuring system, a continuous RF signal is repetitively swept by a frequency modulating signal and the resulting modulated carrier is transmitted towards a target. The FM signal is reflected by the target and returned toward the transmitter for reception. A portion of the transmitted signal is mixed with the reflected signal to produce a beat frequency indicative of the distance from the transmitter to the target. By controlling the modulation of the transmitted signal and by detecting the beat frequency, accurate readings of the distance to a target can be determined. Prior art FMCW systems are described in U.S. Pat. No. 4,107,679 and 4,276,549 and in the article entitled "Area Target Response of Triangularly Frequency-Modulated Continuous-Wave Radars, I.E.E.E. Transactions on Aerospace and Electronic Systems, Volume AES-14, No. 2, March 1978.
While each of the above techniques can give an indication of distance to a target, each technique has its own limitations. For example, the pulsed radar technique enables easy detection and tracking of the nearest return, thereby providing easy detection of altitude or height changes when used as an altimeter. The pulsed technique, however, requires high transmitter power and large receiver bandwidth and is normally confined to a specific frequency which enables easy jamming in a hostile environment. In addition, the high power solid state RF devices which are needed to generate the high power RF pulses are not as readily available as lower power devices. Further, at lower altitudes, it is difficult to generate and detect the pulses necessary to make accurate readings. In automatic landing systems, this becomes a problem since accuracy at low altitudes is essential for proper control.
In contrast, the FMCW technique enables distance measuring with a low power transmitter and narrow bandwidth receiver. Because the signal is continuously modulated or swept in frequency, it is less likely to be affected by jamming techniques. In addition, the cost and complexity is substantially reduced by the ready availability of low power solid state RF devices. By relying on the beat frequency produced by the mixing of the transmitted and received signal, however, the accuracy of the measurement is reduced. Specifically, the beat frequency produced by FMCW returns is not a single frequency but a spectrum of beat frequencies indicative of many reflections received from a target area and representing different altitudes from that target area. Conventional FMCW detection circuits produce readings which are the average of the spectrum of beat frequencies or the centroid of the spectral return of those beat frequencies rather than the nearest return. While this averaging is helpful in automatic landing systems, it does not provide high resolution for ground or terrain avoidance systems where pop-up targets are likely to occur and does not enable the system to respond quickly to rapid changes in altitude.
In particular, the inability of the system to identify pop-up targets during tracking and for reacquiring altitude tracking after rapid changes in terrain or altitude level increases the risk during flight situations. Since conventional FMCW altimeters rely on the average of a spectrum of beat frequencies, any measurement or response to a pop-up target will be severely inaccurate or delayed and any rapid change in altitude or terrain will necessitate lengthy time delays until the system adjusts the modulation slope sufficient to accommodate a new altitude. Conventional systems are, by design, inherently slow in responding to those changes in altitude and terrain. Also, while in such systems it is highly desirable to provide an indication of altitude rate (rate of change of altitude), the altitude rate is normally derived by differentiating altitude. This process is inherently noisy and requires smoothing circuits which necessarily introduce large lags in time response.
In the prior art, a variety of systems have been developed to improve the accuracy of the distance measuring provided by FMCW apparatus. These techniques attempted to improve frequency discrimination and modulation non-linearities in order to more accurately control the beat frequencies. Other techniques attempted to sweep over a range of beat frequencies and selectively filter frequencies in order to more accurately determine individual frequencies indicating nearest returns. Such attempts, however, have met with limited success since the filter characteristics and sweep time for altitude determination are prohibitive over the desired range of altitudes. As a result, the FMCW technique has been used primarily for automatic landing systems and low altitude measurements, while the pulsed technique has been used at high altitudes or where accurate target distance is required. The only other effective compromise has been to include both a pulsed radar and an FMCW radar to obtain the benefits of each during anticipated use.
As can be seen, if the accuracy of an FMCW radar can be improved to allow searching for nearer returns while tracking and to provide improved measurements of altitude rate, its use in more environments would be facilitated. The present invention has therefore been developed to improve that accuracy and to overcome the limitations of the above known and similar techniques.